The present invention is related to a power converter, and more particularly, the present invention is related to a frequency modulation circuit which implements the input-current shaping function at the central tap of the transformer in a power supply.
With the increasing awakenings to the importance of environmental protection, several regulations regarding the harmonics generated by the power supply are prescribed by the known EN 61000xe2x80x943-2 specification. In order to abate the harmonics generated by the power supply, the power supply designer has made a great number of attempts to find ways to adjust the input current waveform of the capacitive filter circuit in the power supply, such that it could closely approach a sinusoidal wave. Because the active power factor corrector is used to adjust the input current waveform to improve its power factor, it is applicable to abate the harmonics of the power supply so as to comply with the requirements of the EN 61000-3-2 specification. However, because the active power factor corrector is quite complicated in circuit design and is highxe2x80x94priced, the soxe2x80x94called singlexe2x80x94stage singlexe2x80x94switch inputxe2x80x94current-shaping (S4ICS) technique is thereupon presented. An example of the S4ICS circuit is shown in the center-tapped circuit including L1, L2, D1, D2 and T1 in FIG. 1 (wherein L denotes inductor, D denotes diode and T denotes transformer). Nevertheless, the low-cost S4ICS circuit also introduces another problem to the power supply designer at the same time. As indicated in FIG. 1, the conducting angle of the bridge rectifier 12 can be effectively magnified by means of the on/off operations of the diodes D1 and D2, thus enhancing the power factor of the input power. When the diode D2 is on, the input current is not filtered by the filtering capacitor Cb, but is directly connected to the bridge rectifier 12. Because the input voltage is lower than the voltage across the filtering capacitor Cb at this moment, the switching current will be larger and therefore the electromagnetic interference (EMI) will get more serious. As a result, an EMI filter of a better quality must be employed to the power supply, which prevents the cost of the power supply from decreasing.
Virginia Power Electronics Center (VPEC) has addressed an essay entitled xe2x80x9cReduction of Power Supply EMI Emission by Switching Frequency Modulationxe2x80x9d at the VPEC Tenth Annual Power Electronics Seminar in 1992, and in that essay it presents a solution to effectively planarize the frequency spectrum, distribution of the noise. A circuit design scheme which makes use of the intrinsic characteristics of the S4ICS circuit to integrate the frequency modulation circuit into the EMI solution based on the S4ICS circuit is substantially proposed in the present invention.
The present invention provides a frequency modulation circuit for a power converter, especially for a switched AC/DC converter provided with input-current shaping (ICS) function. The ICS function is carried out by adjusting the conducting angle of the input current of the rectifier in virtue of the central tap of the main transformer to obtain a higher power factor. Owing to the actions of the ICS, the ripples of the feedback signal will certainly be larger. The present invention utilizes the ripples of the feedback signal to modulate the switching frequency of the power supply, so as to effectively disperse and planarize the energy distribution of the power supply and suppress the EMI problems of the power supply.
The switched AC/DC converter provided with ICS function as described hereinbefore broadly comprises a rectifier and a center-tapped transformer, in which one end of the primary winding of the center-tapped transformer is coupled to a storage capacitor, and the other end is coupled to a switch. The central tap of the center-tapped transformer is coupled to an input-current shaping circuit. The input-current shaping circuit, the rectifier, the storage capacitor and the central tap of the center-tapped transformer form an electric connection to allow the central tap of the center-tapped transformer to accomplish the input-current shaping function.
The switched AC/DC converter provided with ICS function as described hereinbefore further comprises a feedback circuit which outputs a feedback signal to a pulse width modulation control circuit and a frequency-sampling circuit according to the magnitude of the output load of the converter. The frequency-sampling circuit has one end coupled to the feedback circuit and another end coupled to a frequency-generating circuit. The switched AC/DC converter provided with ICS function in accordance with a preferred embodiment of the present invention comprises a frequency-generating circuit which outputs a corresponding frequency value to the pulse width modulation control circuit in response to a signaling level of the feedback signal. The pulse width modulation control circuit controls the on/off operations of the switch in response to the feedback signal from the feedback circuit. The energy of the converter then can be successfully conveyed from the primary winding of the center-tapped transformer to other windings by way of the on/off operations of the switch.